However, the recipe calls for baking soda, and I cook from scratch fairly rarely, so I'm worried the leftover bicarb will go strange in my cupboard. And since people use it for all sorts of purposes, it's one of those products generally not available in the small quantities called for in cooking.

If I do buy a packet, how long is an expected shelf-life for it? Based on this question I probably should find an airtight container to store it in, which may assist.

(Note if relevant: I live in Sydney Australia; Christmas here is mid-summer, and often fairly high-humidity and 35-40 degree C heat.)

6 Answers
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StillTasty provides a very conservative answer: 6 months at best quality. In my personal experience, it can last much longer and be just fine, though I think the longest I ever kept a box was a couple years, not 5 years like riotburn. StillTasty suggests that the concern after more than 6 months is lost potency, but I believe the potential to take on odors from the surroundings is much more of a concern; store it in something airtight like you said, and you should be pretty safe from that.

So I'm sure it'll last at least until next Christmas. And if you do get into baking, you might go through a box faster than you'd think. Otherwise, it's fortunately pretty cheap, and it does have other uses (e.g. cleaning) so it won't hurt you too much to just go for it.

What have you been using the baking soda for? After 5 years, I'd wonder how much leavening power it has.
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KatieKNov 9 '11 at 19:08

Baking soda starts out as about 99% pure sodium bicarbonate. After 5 years on the shelf, interaction with atmospheric components will have the purity down to about 99%. It is the sodium bicarbonate which provides the leavening power. So baking soda will have the same leavening power new, and after 5 years.
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Wayfaring StrangerDec 19 '11 at 23:51

A chemical will behave differently when exposed to air compared with its original state. Plain old baking soda isn't simply dug up - it is manufactured; one way is by dissolving soda ash in water and then bubbling that with carbon dioxide.
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KatieKNov 9 '11 at 19:44

@KatieK that depends on where you live. "Natron" has been dug up in African and Middle Eastern areas for millennia. In the USA, they have been mining it for years in Colorado etc
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TFDNov 10 '11 at 10:24

Sodium bicarbonate is stable at room temperature. Above 70 or 80 degrees C, it will degrade into sodium carbonate, water and carbon dioxide. Keep it in a sealed container (esp away from the oven and acids such as vinegar) and it should last indefinitely.

Even though the baking soda won't decompose into hunks of salt and carbon within a year, it will lose the leavening power (potency and performance) that many baking applications (such as in cookies and quickbreads) rely on.

Baking is chemistry. To get the same outcome as the recipe intends, you need to use precisely the same proportions and ingredients as the recipe states. If the baking soda you use doesn't give the same rising power (leavening) at the same times as the recipe expects, then you will have different results.

Someresources suggest testing the efficacy of baking soda by putting some in water or vinegar and looking for fizzing. You can do this at home to see a difference between old and new baking soda. But there is no way quantifiable to quantify how much fizz occurs. Without quantifying the decrease in efficiency, you have no way of knowing how much more baking soda you might need to get the expected result.

When I bake cookies with baking soda older than 6 months, they are definitely flatter than when I use newer baking soda. Here are some cookies that I baked with new baking soda, and baking soda from last year.

I realise you're asking about baking soda, but not being happy with the rise I was getting on American-style pancakes, I bought some new baking powder and instantly got that good ol' rise back. I'd had the previous baking powder for 6 months and it still had 6 months until expiry. I now keep the new powder in an airtight container now, not the cardboard cylinder it came in.

I did the fizz test KatieK talks of (but I think I used lemon juice), and the new powder easily fizzed twice as much.

It's dirt cheap and has so many other uses, I just wouldn't take the chance.